1. Field of the Invention
This invention relates to an illumination optical system for an ultraviolet microscope apparatus which uses wavelengths in an ultraviolet region and to an optical filter for obtaining only light with desired wavelengths in the ultraviolet region, and in particular, to an illumination optical system and an optical filter for an ultraviolet microscope apparatus which uses wavelengths in a deep ultraviolet region, less than 300 nm. This invention also relates to an illumination optical system for an ultraviolet microscope apparatus which is used in combination with a conventional microscope optical apparatus for making observations with wavelengths in a visible region.
2. Description of Related Art
In keeping with the development of high-density Integrated Circuits (ICs), microminiaturization of wiring patterns is rapidly advanced, and thus there are strong and growing demands that optical microscopes used for their observations and examinations should have high resolution. As is well known, there are two techniques of obtaining high resolution in an optical microscope. One is to increase the numerical aperture of an objective lens, and the other is to shorten wavelengths used. However, since under the present situation the numerical aperture already reaches a limited value as high as 0.95, it is very difficult to desire a higher numerical aperture. Hence, the technique of reducing wavelengths must be used, but with wavelengths in the visible region, it is difficult to accommodate microminiaturization of recent ICs, and it becomes necessary to use ultraviolet rays which have wavelengths shorter than those in the visible region.
Various ultraviolet microscopes have been suggested, and examples of ultraviolet microscopes using light sources, such as mercury-vapor lamps, are set forth in Japanese Patent Preliminary Publication Nos. Sho 64-62609 and Hei 5-127096. A microscope disclosed in Sho 64-62609 is designed so that all lenses placed in optical paths of ultraviolet rays are constructed of quartz. On the other hand, a microscope disclosed in Hei 5-127096 is such that an illumination lens and an objective lens system are corrected for chromatic aberration in wavelength regions extending from a visible region to a near-ultraviolet region, and is equipped with means for observing a visible image and an ultraviolet image.
A light source used for illumination has a specialty with respect to each of the characteristics of its spectral intensity distribution and emission point. For an illumination optical system taking account of the characteristic of a special emission point of the light source itself, it is disclosed in Japanese Patent Preliminary Publication No. Hei 6-289301 that a nearly afocal, variable-magnification lens system is interposed to be removable between a lamp house containing a light source and collector lenses and a projection optical system. This variable-magnification lens system is provided for the purpose that when a halogen lamp is replaced by an arc lamp, the magnification of projection of the lamp on the pupil of an objective lens is changed. The lens system includes two lens units with two lens components, comprised of a positive lens and a negative lens which are arranged in this order from the collector lens side.
Here, a brief description of the arc lamp is given. An arc lamp, like a high-pressure mercury-vapor lamp, is adapted to cause an electric discharge between an anode and a pointed cathode so that extremely bright light is produced. In the arc lamp, a space between the anode and the cathode is narrow, and the discharge is not maintained at a uniform intensity between them. Truly high luminance is kept only in a very small region close to the pointed cathode. Thus, in Hei 6-289301, when the light source is switched to the arc lamp, the variable-magnification lens system is inserted to increase the magnification of projection of an arc on the pupil of the objective lens, thereby casting bright illumination upon the pupil of the objective lens.
In order to make observations with ultraviolet rays, it becomes necessary to use an optical filter for selecting only light in a particular wavelength region from among various wavelengths emitted from a light source. In a conventional, optical microscope used in a visible region, an optical filter (namely a band-pass filter, which is hereinafter abbreviated to BPF) placed in an illumination optical system or an observation optical system to obtain only light in a desired wavelength region, as shown in FIG. 1, is such that its spectral transmittance characteristic curve is symmetrical about a wavelength xcexc in the vicinity of the desired wavelength region. Hence, this spectral transmittance distribution is not set to take account of a special, spectral intensity distribution characteristic of the light source itself.
Another technique of obtaining light with desired wavelengths in an ultraviolet wavelength region is that, in a microscope illumination system using deep ultraviolet light from a light source such as an arc lamp, light other than desired deep ultraviolet light is transmitted through a cold mirror and only deep ultraviolet light reflected by the cold mirror is used.
For still another technique, an example of a combination of a transmission-type element and a reflection-type element is disclosed in Japanese Patent Preliminary Publication No. Hei 8-313728.
In the case where the technique disclosed in Sho 64-62609 is employed, however, lens material used is limited to quartz, and therefore correction for chromatic aberration is impossible. Thus, there is the problem that a substantially usable wavelength is restricted to a certain particular wavelength (or a very narrow wavelength region). Moreover, because chromatic aberration cannot be corrected, the resolution and contrast of an image may be degraded due to the chromatic aberration unless light with wavelengths other than this wavelength is cut off by a filter. In the illumination optical system, aberration also causes uneven illumination. Because the usable wavelength is limited to the above wavelength, only a dark image can be observed. Furthermore, this prior art publication fails to give a description of a combination with a conventional microscope for visible-region observation, and thus the possibility of this combination is unclear. Since a specific means or condition for selecting wavelengths in the ultraviolet region, shorter than 300 nm, is not described, the optimum resolution, contrast, and brightness cannot be obtained directly even if the technique is used as it is.
In contrast to this, with the technique disclosed in Hei 5-127096, chromatic aberration is corrected in the range from the visible region to the near-ultraviolet region, and hence it is possible to combine a visible-image observation with an ultraviolet-image observation. Since, however, chromatic aberration is not corrected in the region of wavelengths shorter than 300 nm, the resolution and contrast of an image in this wavelength region are considerably deteriorated and uneven illumination is produced. In addition, this publication provides a means for separating a visible image from an ultraviolet image, but fails to suggest a specific means or condition for selecting wavelengths in the ultraviolet region, less than 300 nm. Therefore, the optimum resolution, contrast, and brightness cannot be obtained directly even if the technique is used as it is.
The technique disclosed in Hei 8-313728 is provided for use in semiconductor exposure so that the transmittance of a wavelength used is increased and those of the other wavelengths are decreased. However, since the technique is not directed to photography and observation for which a TV camera is used, this publication does not in any way suggest to what extent the transmittances of the other wavelengths must be held or a consideration for the transmittances of wavelengths in the infrared region. Moreover, an illumination optical system and an imaging optical system are not specifically described.
Where the ultraviolet rays are employed, a TV camera must be used for observation and examination, but a resulting image is black and white and does not contain color information. However, an actual examination may be made on color information for which light in the visible region is used, and hence it is desired to carry out both the ultraviolet-region observation in which high resolution is obtained and the visible-region observation in which color information is derived. Each of the above prior art publications, however, fails utterly to specifically prove the possibility of such a technique.
On the other hand, for the optical filter, the BPF is such that as the slopes of the ascent and descent of its transmittance characteristic curve are steepened, its design and fabrication become difficult. In general, the BPF is multilayer-coated with interference films having a short-wavelength cutoff characteristic and a long-wavelength cutoff characteristic (both cutoff characteristics are almost the same), thereby exhibiting desired characteristics. However, if an attempt is made to steepen the slopes, the kind of film will be increased and the whole number of coating films will also be increased. Consequently, the design of the BPF becomes very difficult. Furthermore, when the BPF is fabricated, its respective film thicknesses must be rigidly controlled because of an increase of the whole number of coating films. Thus, there is the problem that a failure to control the film thickness in any layer causes deterioration in design performance and makes its fabrication very difficult. Since, in an ordinary lamp, light is radiated over a wide wavelength region, the BPF selecting only light in a desired wavelength region therefrom requires that the slopes of the ascent and descent of its transmittance characteristic curve are steepened in the same way on both the short-wavelength side and the long-wavelength side. In this way, the above problem arises on each of the short-wavelength side and the long-wavelength side.
The variable-magnification lens system placed at the middle of the illumination optical system which is disclosed in Hei 6-289301 can be designed to include two lens units with two lens components, comprised of a positive lens and a negative lens, because various glass materials with different refractive indices and dispersion properties can be used for lenses in an illumination optical system using visible light. However, in the region of wavelengths shorter than 300 nm, materials used for lenses, from the viewpoint of transmittance, are limited to two kinds: fluorite and quartz. With this construction, aberrations including chromatic aberration cannot be corrected.
In the construction that only desired ultraviolet light is substantially used, there is the problem that light in the region of other wavelengths becomes flare or ghost, which adversely affects observations.
It is, therefore, a primary object of the present invention to provide an ultraviolet microscope optical system which, when using a light source emitting light over a wide wavelength range from ultraviolet light to infrared light, is capable of observing an image that is bright and high in resolution and contrast and can be favorably used in combination with the optical system of a conventional microscope for visible-region observation, and which chiefly uses wavelengths in a deep ultraviolet region, shorter than 300 nm.
It is another object of the present invention to provide an optical filter which is easy in design and fabrication and has high efficiency, taking account of characteristics of a light source.
It is still another object of the present invention to provide a reflecting illumination optical system which is favorably corrected for aberrations and projects an arc lamp on the pupil of an objective lens at a sufficient projection magnification.
It is a further object of the present invention to provide a reflecting illumination optical system which is optimized to take advantage of light in a wavelength region which never has been used before.
In order to achieve the above objects, according to one aspect of the present invention, the ultraviolet microscope optical system is constructed so that light in the region of wavelengths shorter than 300 nm, emitted from a light source, irradiates a specimen through an illumination optical system, a path connecting means, and an objective lens system, and the light reflected by the specimen is imaged through the objective lens system, the path connecting means, and an imaging optical system, a resulting image being photographed by a photographing means. A wavelength selective means is placed in the microscope optical system, and the illumination optical system and the objective lens system, as well as the path connecting means and the imaging optical system, are corrected for chromatic aberration in the range of wavelengths selected by the wavelength selective means. When a transmittance at the center of a selected wavelength region is represented by To, a half width at full maximum is represented by xcex4(nm), an average transmittance in the region of wavelengths longer than 300 nm is represented by Tm, and a sensitive wavelength region of the photographing means in the region of wavelengths longer than 300 nm is represented by xcex94(nm), the wavelength selective means satisfies the following condition:
(Toxc2x7xcex4)/(Tmxc2x7xcex94) greater than 2 
According to another aspect of the present invention, the ultraviolet microscope optical system includes a first microscope optical system in which light emitted from a first light source irradiates a specimen through a first illumination optical system and a first objective lens system so that the light reflected by the specimen can be observed through the first objective lens system, a first imaging lens system, and an observation optical system, and a first wavelength selective means is interposed between the first objective lens system and the first imaging lens system to reflect light with wavelengths shorter than 300 nm and transmit light with wavelengths ranging at least 400 nm to 700 nm. The ultraviolet microscope optical system further includes a second microscope optical system in which light in the region of wavelengths shorter than 300 nm, emitted from a second light source, irradiates the specimen through a second illumination optical system, a path connecting means, the first wavelength selective means, and a second objective lens system, and the light reflected by the specimen is imaged through the second objective lens system, the path connecting means, and a second imaging optical system, a resulting image being photographed by a photographing means. A second wavelength selective means is interposed between the second light source and the path connecting means, and the second illumination optical system and the second objective lens system, as well as the path connecting means and the second imaging optical system, are corrected for chromatic aberration in the range of wavelengths selected by the second wavelength selective means. When a transmittance at the center of a selected wavelength region is represented by To, a half width at full maximum is represented by xcex4(nm), an average transmittance in the region of wavelengths longer than 300 nm is represented by Tm, and a sensitive wavelength region of the photographing means in the region of wavelengths longer than 300 nm is represented by xcex94(nm), the second wavelength selective means satisfies the following condition:
(Toxc2x7xcex4)/(Tmxc2x7xcex94) greater than 2 
A unit including the first light source and the first objective lens system or a unit including the second light source and the second objective lens system is selectively used, and thereby a visible image or an ultraviolet image can be selectively observed.
The optical filter according to the present invention is constructed so that its spectral transmittance characteristic curve is asymmetrical when wavelengths are plotted along the abscissas against transmittances along the ordinates.
The reflecting illumination optical system of the present invention includes a combination of a light source causing self-absorption and an optical filter whose spectral transmittance characteristic curve is asymmetrical when wavelengths are plotted along the abscissas against transmittances along the ordinates.
The reflecting illumination optical system of the present invention is provided with a light source, a collector lens for practically collimating a light beam emitted from the light source, a dichroic mirror for reflecting desired deep ultraviolet light from a practically collimated beam and transmitting light extending from the near-ultraviolet region to the near-infrared region, a first illumination optical system which uses a light beam reflected by the dichroic mirror to make observations through an objective lens for deep ultraviolet light, and a second illumination optical system which uses a light beam transmitted through the dichroic mirror to make observations through an objective lens for visible light.
These and other objects as well as the features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.